Elementary Pharmacokinectics Flashcards
What is the minimum effective concentration?
The amount of drug required to give an effect
What is the maximum tolerated concentration?
The concentration where there is significant side effects
What is the therapeutic ration?
the window between the MTC and MEC
TR=MTC/MEC
What are “safe” drugs?
drugs with a high TR - penicillins and BZD
What are unsafe drugs?
drugs with a low TR - cardiac glycosides, barbituates
How is the initial concentration calculated?
Co=D/Vd
What does the concentration depend on over time?
Kel - rate of elimination
What sort of kinetics do most drugs follow?
first order
What equation does drug fall according to?
Ct=Coe^-Kel.t
What is the t1/2 calculated by?
t1/2 = 0.693/Kel
What does increasing the dose do for drugs that exhibit first order kinetics?
increases Cp but does not affect Kel or t1/2
What is clearance?
the volume of plasma cleared of drug in unit time
What does clearance apply to?
drugs that exhibit first order kinetics
What is the equation for rate of elimination?
Rate = Cp x Cl
What does concentration drive?
elimination
What is true at steady state?
rate of drug administration = rate of drug elimination
What is the rate of elimination at steady state?
R=Cl x Cpss
What is clearance the sum of?
all clearance processes i.e. renal, hepatic, other
When is Css reached?
after 5 half lives regardless of route of administration
What is the time to reach Css determined by?
t1/2 but not infusion rate
How do you calculate Css in oral drugs?
Css = F*dose/Clp x dose interval
where F is oral bioavailability
What is the volume of distribution?
the volume into which a drug appears to be distributed with a concentration equal to that of plamsa
What is the equation relating to volume of distribution?
a proportionality constant relating the plasma concentration to the amount of drug in the body (Ab)
Ab=Vd xCp
How would you determine Vd?
at time 0 - Ab = dose
then Cpo = dose/Vd so Vd = dose/Cpo on a graph with log Cp
What is a loading dose?
an initial higher dose of drug given at the beginning of a course of treatment before stepping down to a lower maintenance dose
What is the loading dose used for?
to decrease the time to steady state for drugs with a long half life?
What is the equation for a loading dose using iv?
LD = Vd x Target Cp
What is the equation for a loading dose using oral admin?
LD = Vd x Target Cp/F
What does t1/2 give an index of?
the time course of drug accumulation
the time course of drug elimination
choice of dose interval
What is the equation for t1/2?
t1/2 = 0.693 x Vd/Cl
What are the independent variables when calculating t1/2?
Vd and Cl
What is the elimination pattern in zero order kinetics?
initially eliminated at a constant rate and then convert to first order kinetic at a low concentration
When can zero order kinetics occur?
when the plasma concentration of the drug is higher than the Km of the enzyme which metabolises it
